1. Field of the Invention
The present invention relates to an image sensing apparatus.
2. Description of the Related Art
Recent popular image sensing apparatuses such as a digital camera and digital video camera can capture both still images and movies using image sensors such as a CMOS image sensor.
When capturing a movie with an image sensor, the image sensing apparatus requires an electronic shutter function to control the exposure of the image sensor because it is difficult to use a mechanical shutter. The electronic shutter function controls the shutter speed by changing the charge accumulation time of pixels in accordance with the brightness of an object to accumulate a signal at an appropriate level. The electronic shutter function includes a simultaneous electronic shutter function and slit rolling electronic shutter function, as described in Japanese Patent Laid-Open No. 2007-028337.
Some image sensing apparatuses of this type can interrupt movie shooting to perform still image shooting. In the movie shooting mode, the image sensing apparatus records a movie continuously captured by an image sensor, and displays it on a display such as an LCD (Liquid Crystal Display). In the still image shooting mode, the image sensing apparatus displays a movie continuously captured by the image sensor on the display without recording it. Upon receiving a still image shooting instruction via a shutter button or the like in the still image shooting mode, the image sensing apparatus suspends an electronic viewfinder function used in movie shooting, and executes still image shooting.
Required Performances differ between still image shooting and movie shooting. Still image shooting requires high spatial resolutions, while movie shooting requests high temporal resolutions.
More specifically, the number of pixels requested of still image shooting often exceeds 10,000,000 pixels, while the number of pixels requested of movie shooting is about 300,000 pixels or 2,000,000 pixels at most even for the full high-vision (HDTV) standard. The frame rate necessary for still image shooting is not particularly limited, while the frame rate necessary for movie shooting is about 30 frames/sec or 60 frames/sec.
To read out signals of as many pixels as requested of still image shooting from a pixel array at a frame rate necessary for movie shooting, the image sensor needs to be driven at very high speed, consuming a high power. To avoid this high power consumption, the movie shooting mode often adopts a method of thinning out pixels vertically or horizontally from the pixel array of an image sensor and reading out signals from the remaining pixels because the necessary number of pixels is smaller than that of a still image.
Some image sensing apparatuses can perform still image shooting parallel to movie shooting.
Assume that a still image is formed from one frame of a movie obtained by thinning-out readout in order to perform still image shooting parallel to movie shooting. In this case, still image data generated from a movie image does not satisfy the number of pixels requested of still image shooting. A still image obtained from the generated still image data is poor in spatial resolution.
Assume that the movie frame rate is temporarily decreased upon receiving a still image shooting instruction, and signals are read out from all the pixels of the pixel array of an image sensor in order to execute still image shooting parallel to movie shooting. In this case, movie data is generated at the temporarily low frame rate. A movie obtained from the movie data is poor in temporal resolution.
To solve this, Japanese Patent Laid-Open No. 2005-012403 discloses a technique of reading out signals from rows which change by thinning-out between a predetermined number of successive frame periods, and generating the signals of the rows read out in the predetermined number of frame periods. According to Japanese Patent Laid-Open No. 2005-012403, still image data of one frame can be generated from the signals of pixels on all the rows of the pixel array. A still image with high spatial resolution can be attained without decreasing the movie frame rate.
Japanese Patent Laid-Open No. 2000-134549 discloses a technique of reading out signals from pixels which change by sampling between a predetermined number of successive frame periods, and composing the signals of the pixels read out in the predetermined number of frame periods, thereby reconstructing a still image. According to Japanese Patent Laid-Open No. 2000-134549, still image data of one frame can be reconstructed from the signals of all the pixels of the pixel array. A still image with high spatial resolution can be provided without decreasing the movie frame rate.
As described above, in the techniques disclosed in Japanese Patent Laid-Open Nos. 2005-012403 and 2000-134549, still image data of one frame is generated by composing the signals of different pixels that have been accumulated in different frame periods. This may impair the simultaneity of accumulation of 1-frame still image data between pixels. Especially when an object is moving, the object position may change between pixel rows or pixels. The spatial resolution of an obtained still image may substantially lower.